1. Field of the Invention
The invention is generally related to electrical switches which use a conducting fluid to selectively provide a conductive bridge between the space separating two electrodes and, more particularly, to non-toxic substitutes for mercury (Hg) which have similar performance characteristics in switch applications.
2. Description of the Prior Art
Mercury is used extensively in switches and sensors. In a common switch application, liquid mercury is positioned inside a fluid tight housing into which a pair of spaced apart electrodes extend. Depending on the physical orientation of the housing, the liquid mercury can provide a conductive pathway between the electrodes or be positioned such that there is an open circuit between the electrodes. An important physical attribute of mercury metal is that it remains fluid throughout a wide temperature range, and can therefore be used in many different environments and in environments with constantly changing parameters. Another important physical attribute of mercury metal is that it has significant surface tension and does not wet many glass, polymer or metal surfaces.
Mercury metal is sufficiently toxic that human and animal exposure is a significant concern in any application or process in which it is used. Concentrations as low as 0.03 mg/m.sup.3 have induced psychiatric symptoms in humans. Mercury has also been identified as disrupting the endocrine system in certain wildlife and possibly in humans. Utilization of mercury during manufacturing may present a health hazard to plant personnel, and the disposal of devices that contain mercury switches or the accidental breakage of mercury switches during use may present indirect hazard to people within the immediate vicinity of the switch.
Japanese Patent Application Sho 57-233016 to Inage et al. discloses a metallic alloy which includes 75% gallium, 19-29% indium, 1-11% tin, and 1-3.5% silver, and discusses its usefulness as a possible substitute for mercury. This alloy has low toxicity, is not very volatile at normal temperatures, has a temperature of congealment below 0.degree. C., and has a resistance of 32 .mu..OMEGA./cm at 20.degree. C. This patent also identifies Japanese Patent Disclosure Sho 50-101208 as describing a gallium based alloy which includes indium, tin, zinc, silver, and aluminum.